This invention relates to battery backup systems having a large number of battery cells and, more particularly, to the measurement of battery cell impedance while the batteries remain on-line.
Large battery systems are commonly used to provide backup power in case there is a failure of the commercial power grid. Typically, such a backup system includes a single string or a plurality of parallel strings of serially connected rechargeable battery cells and a charger connected to the commercial power grid for maintaining the charge on the battery cells. An inverter is coupled between the strings of battery cells and the load, which inverter is enabled upon the detection of a failure of the power grid. In some applications, the inverter may be continuously operational to power the load with energy from the charger during the time that commercial power is available. Many of these battery backup systems, called "uninterruptable power supplies", are configured such that the load is never aware of any failure of the power grid because the battery system immediately supplies the necessary energy upon failure of the power grid.
A typical installation of such an uninterruptable power supply is between the power grid and a large computer system used by financial, communications, manufacturing and other commercial industries. If the battery system is taken "off-line" for any reason, the necessary protection against power outages is lost for the time that the battery system is not connected plus the time for recharging, if a significant amount of charge has been removed during the off-line period of time. However, such battery backup systems must be monitored on a regular basis to insure that protection from power grid failure is always available.
Impedance measurement is a method by which the condition of a battery may be assessed without taking the battery off-line. Impedance measurements typically impose a current (hereinafter called the "loading current") on the part to be evaluated and measure the resultant voltage. Various commercially available test instruments function this way. Using Kelvin connections, these instruments impose a current on just the component to be measured. After a measurement has been made, the operator moves the Kelvin clips to the next component, reads the value, moves the clips again, and continues in this manner until all the components have been measured. Therefore, the loading current flows almost entirely through the component being measured, the parallel paths (if they exist) generally being of so much higher impedance that any loading current flowing through them is of little or no consequence.
U.S. Pat. No. 5,047,722 discloses a system for measuring the impedance of a battery cell within a string of battery cells. The disclosed system draws a pulsed, measured or controlled loading current from the entire string and, while doing so, measures the voltage across each of the cells or groups of cells which make up the battery string. However, if the output impedance of the charger is extremely low compared to the impedance of the battery at the measurement frequency, or if the battery is composed of multiple parallel strings, then only some part of the loading current being drawn to make the measurement will be flowing through the cell or interconnection being tested. If this is the case, the calculated impedance may be less than the true value. In the extreme case, generally when the impedance of the charger is very low (the practice of placing a large capacitance across the charger output terminals to protect the battery from ripple currents is becoming increasingly prevalent with uninterruptable power supply manufacturers) , such a small percentage of the loading current comes from the battery that impedance measurement is virtually impossible. Of course, when a low output impedance charger is used, the charger is the dominant shunt impedance, so that the presence of any number of parallel strings of cells is of no consequence.
It is therefore an object of the present invention to be able to measure battery cell impedance in a battery backup system of the type described which obviates all of the described problems which result from low charger impedance and the existence of multiple parallel strings.